WASHINGTON, 11 April 2003 — Although Defense Secretary Donald Rumsfeld warned Wednesday that the war in Iraq isn’t over, the US military has already achieved some of its major objectives in just 21 days: a relatively small force has seized nearly 80 percent of a country the size of California, including its capital, while suffering just over 100 dead.

Military professionals attribute the US success to three key elements: a seasoned and well-equipped military, a surprisingly inept Iraqi response, and the decision of the Bush administration at the end of March, when the US and British attack seemed to be faltering in southern Iraq, to keep the army and Marines focused on Baghdad.

The fighting isn’t over yet. Small-scale actions are likely to continue for a week or two, and there still could be major battles in the north. Indeed, administration officials were wary of declaring the war finished. “It most assuredly is not over,’’ Rumsfeld said at a Pentagon news conference. “There’s going to be some very tough days ahead ‘’

Even so, Rumsfeld did allow himself to acknowledge that, “I think it’s safe to say that the mood in the country is, in fact, tipping, at least in Baghdad.’’

“We won so handily because we had a highly professional military — well-trained, well-outfitted, well-led, with the right doctrine, solid organization, and, most important, excellent people,’’ said retired army Col. James McDonough.

With some reinforcements arriving in recent days, the US invasion force in Iraq still totals only 125,000, a fraction of the half-million troops assembled for the 1991 Gulf War.

In particular, in this war, more than in any previous one, air and ground operations seemed thoroughly meshed, with targeting information between US special forces and pilots flowing back and forth as it never has before.

The disadvantage of precision weapons, US officers have noted in previous wars, is that to be used effectively they require precise information. In Iraq for the past three weeks, troops operated in such a way as to find and move that data. Special Operations forces spotted targets and quickly transmitted the information into a system that moved it to bombers orbiting overhead. Reconnaissance drones loitered over the battlefield, showing US commanders enemy movements in real-time video.

When asked Wednesday why the US military moves with more agility in combat now than it did during the 1991 Gulf War, a Pentagon official said, “Everything that has a sensor is connected.’’

The next element was the war plan the commanders followed. “It was an audacious plan, even a risky one,’’ said retired air force Lt. Col. Rick Francona, an expert on the Iraqi military. It flung US forces deep into Iraqi territory at the end of unsecured supply line that suffered repeated ambushes and sniping attacks. It didn’t provide for armored reinforcements. And to an unprecedented degree it counted on air power to help out any ground units that got into trouble.

It also was a demanding plan, one that couldn’t be executed by slow-moving commanders or poorly trained troops. Pentagon officials cited the plan’s “flexibility’’ so incessantly that it threatened to become a cliché, but the term actually is meaningful in a military sense. Only well-trained forces can quickly change plans, accelerating their movements to take advantage of newly discovered opportunities.

Such last-minute alterations happened several times during the first three weeks of the war, most notably with the ground war being kicked off 24 hours before planned. When the 3rd Infantry Division arrived at the outskirts of Baghdad, rather than ring the city and wait, its commanders decide to pummel Iraqi forces in the capital with armored raids that first established US control over the international airport and, over the last five days, much of the rest of the city. Similarly, members of the 1st Marine Expeditionary Force threw out their plans Wednesday for a slow movement into Baghdad from the east and, with Iraqi authority evaporating, instead drove straight downtown.

The most important aspect of the execution of the plan, said one person who helped develop it, wasn’t anything that commanders in the field did, but rather the determination of the Bush administration to stick with it. It was, he said, an approach “that they stuck to even as the US press and some parts of the military, and especially the heavy army, were doing an outstanding imitation of Chicken Little.’’

At the end of March, with US and British forces being subjected to guerrilla-style attacks in cities along the invasion route from Kuwait to Baghdad, there was some question inside and outside the US government about whether the Pentagon should turn its attention to consolidating the gains in the south before advancing on the capital. On March 29, President Bush convened a teleconference call from Camp David with his senior national security advisers and decided to keep the military’s sights fixed on Baghdad, calculating that the Iraqi capital remained the primary objective in the war.

Randy Scheuneman, president of the Committee for the Liberation of Iraq, an anti-Saddam lobbying group, cited that decision as pivotal. “I think the turning point in the war was the decision to ignore the cacophony of criticism and stick with an aggressive plan based on taking Baghdad rather than liberating Iraq sequentially,’’ he said.

Following that decision, the overarching instruction that Marine Maj. Gen. James Mattis, commander of the 1st Marine Division, gave his subordinates was “speed, speed, speed,’’ said one Marine.

That final leap to Baghdad was in some ways the most difficult move of the ground war to date. US commanders already were worried about their strained and vulnerable supply lines. And they expected to be confronted by the Iraqi military’s best fighters, the Republican Guard, and perhaps to be attacked with chemical weapons.

“It was a big swallow for the land forces to move out with unsecured flanks and a tenuous logistic supply line,’’ said retired air force Gen. Charles Horner, the commander of the air campaign in the 1991 war against Iraq. “But that kept the Iraqis off guard.’’

That three-day push from southern Iraq to the outskirts of Baghdad during the first three days of April, said Winslow Wheeler, a fellow at the Center for Defense Information, a Washington think tank that is frequently critical of Pentagon policy, “was truly remarkable.’’

He predicted it will be compared by historians to Stonewall Jackson’s Virginia campaign in the Civil War and George Patton’s march across France in World War II.